Kyoukaisenjou no Horizon

In the distant future, humanity abandoned a devastated Earth and ascended to a celestial realm known as Tenjo, where they assumed god-like forms. However, divine warfare eventually shattered both Tenjo and the fundamental laws of causality. Stripped of their powers, humans were forced to return to an uninhabitable Earth, finding refuge only in a region corresponding to modern-day Japan, which they renamed Shinshuu, or the Divine States. Because this territory was insufficient to sustain the entire population, they created a parallel dimension called the Harmonic Realm, establishing duplicate lands known as the Harmonic Divine States. Guided by sacred records called the Testament, humanity began reenacting history from 10,000 BC to restore Tenjo and repair the broken laws of existence. During the Testament Era year 1413, the historical reenactment of the Nanboku-cho War within Shinshuu accidentally damaged the barriers separating the dimensions, causing the Harmonic Realm to collapse into the original world. Displaced and landless, the inhabitants of the Harmonic Divine States launched an invasion of Shinshuu. The native population was defeated, and under the terms of surrender, their territory was partitioned among the invaders while discriminatory laws were enacted against them. Although the historical reenactment resumed from 1457 AD, updates to the Testament abruptly ceased at 1648 AD, sparking widespread rumors of an impending apocalypse. Amidst this turmoil, the floating city of Musashi, composed of eight massive vessels, drifts through the skies above a fractured Japan now divided into feudal territories controlled by foreign powers. It is upon this aerial bastion that students must wage a war that will ultimately decide the fate of the world.